The local structures of electrochemically deposited Cu on p-GaAs(100) of various coverages in 0.1 M H2SO4 solution containing 0.1 mM CuSO4 have been investigated by in situ surface-sensitive X-ray absorption fine structure (XAFS). The Cu K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) were obtained over a wide range of coverages, i.e., several monolayers down to 1/20 monolayer. The results of multishell parameter fitting of EXAFS data showed that Cu nanoclusters with a short Cu-Cu bond length were formed on the p-GaAs(100) electrode below the critical surface coverage of 0.25 monolayer. The nanoclusters were found to be coordinated with the oxygen atoms of water molecules and/or sulfate anions. They were formed in an early stage of electrochemical deposition and grew in size to approach a densely packed coordination with an fee structure. XANES spectra showed that the coordination number changed systematically but that a metallic state was retained throughout the electrochemical deposition.